ch09 benzene and deriv

9-1

WILLIAM H. BROWN

THOMAS POON

www.wiley.com/college/brown

C H A P T E R N I N E

Benzene and Its Derivatives

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

9-2Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Benzene - Kekulé

• The first structure for benzene was proposed by August Kekulé in 1872.

– This structure, however, did not account for the unusual chemical reactivity of benzene.


Benzene – Orbital Overlap Model

• The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure.– The carbon skeleton is a regular hexagon, with all C-

C-C and H-C-C bond angles 120°.


Benzene - Orbital Overlap Model

– (a) The carbon framework; the six parallel 2p orbitals, each with one electron, are shown uncombined.

– (b) Overlap of the six 2p orbitals forms a continuous pi cloud, shown as one torus above the plane of the ring, the other below it.


Benzene - Resonance Model

• We often represent benzene as a hybrid of two equivalent Kekulé structures.– Each Kekulé structure makes an equal

contribution to the hybrid.– The C-C bonds are neither double nor single

but something in between.


Benzene - Resonance Energy

• Resonance energy: The difference in energy between a resonance hybrid and its most stable hypothetical contributing structure in which electrons are localized on particular atoms and in particular bonds.– One way to estimate the resonance energy of

benzene is to compare the heats of hydrogenation of benzene and cyclohexene.



• Heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated).

– These results are shown graphically on the next slide.



– Figure 9.2 The resonance energy of benzene.


Resonance Energy

– Resonance energies [kJ/mol and kcal/mol] for benzene and several other aromatic hydrocarbons.


Concept of Aromaticity

• The criteria for aromaticity were recognized in the early 1930s by Erich Hückel.

• To be aromatic, a ring must:– have one 2p orbital on each atom of the ring.– be planar or nearly planar, so that overlap of all 2p

orbitals of the ring is continuous or nearly continuous.– have 2, 6, 10, 14, 18, and so forth pi electrons in the

cyclic arrangement of 2p orbitals.

• Benzene meets these criteria– It is cyclic, planar, has one 2p orbital on each atom of

the ring, and has 6 pi electrons (the aromatic sextet) in the cyclic arrangement of its 2p orbitals.

H3C CH C CH CH3

9-11

Huckel’s Rule

4n + 2 = number of pi electrons If n = integer, follows the Huckel’s rule

If n is a fraction, does not follow the Huckel’s rule

Ex 4n + 2 = 6 pi electrons

4n = 6 – 2

n = 4 = 1

4

4n + 2 = 8 pi electrons

n = 8 – 2 / 4 = 6/4 = 1 ½



Heterocyclic Aromatics

• Heterocyclic compound: A compound that contains one or more atoms other than carbon in its ring.

• Heterocyclic aromatic compound: A heterocyclic compound whose ring is aromatic.– Pyridine and pyrimidine are heterocyclic analogs of

benzene; each is aromatic.


Pyridine

– The nitrogen atom of pyridine is sp2 hybridized.– The unshared pair of electrons lies in an sp2 hybrid

orbital and is not a part of the six pi electrons of the aromatic sextet.

– Pyridine has a resonance energy of 32 kcal (134 kJ)/mol, slightly less than that of benzene.



– Figure 9.3 Origin of the six pi electrons (aromatic sextet) in furan and pyrrole.



• Nature abounds in compounds with a heterocyclic aromatic ring fused to another aromatic ring.


Nomenclature

• Monosubstituted alkylbenzenes are named as derivatives of benzene.– Many common names are retained.


Nomenclature

• Phenyl and benzyl groups


Nomenclature

• Disubstituted benzenes– Locate substituents by numbering or– Use the locators ortho (1,2-), meta (1,3-), and para

(1,4-)

• Where one group imparts a special name, name the compound as a derivative of that molecule.


Nomenclature

• Polysubstituted benzenes– With three or more substituents, number the atoms of the

ring.– If one group imparts a special name, it becomes the

parent name.– If no group imparts a special name, number to give the

smallest set of numbers, and list alphabetically.


Polynuclear Aromatic Hydrocarbons

• Polynuclear aromatic hydrocarbons (PAHs)– Contain two or more fused aromatic rings.


Carcinogenic PAHs

• Benzo[a]pyrene is a carcinogen.– Once absorbed, the body oxidizes it to a more

soluble compound that can be excreted.

– The diol epoxide contains a reactive epoxide ring and can bind to DNA, thereby altering the structure of DNA and producing a cancer-causing mutation.


Benzylic Oxidation

• Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4.– Halogen and nitro substituents are unaffected

by these reagents.– An alkyl group with at least one hydrogen on

the benzylic carbon is oxidized to a carboxyl group.


Benzylic Oxidation

– If there is more than one alkyl group, each is oxidized to a -COOH group.

– Terephthalic acid is one of the two monomers required for the synthesis of poly(ethylene terephthalate), a polymer that can be fabricated into Dacron polyester fibers and into Mylar films.


Reactions of Benzene

• The most characteristic reaction of aromatic compounds is substitution at a ring carbon.


Reactions of Benzene


Electrophilic Aromatic Substitution

• Electrophilic Aromatic Substitution (EAS): A reaction in which an electrophile, E+, substitutes for an H on an aromatic ring.

• We study– several common types of electrophiles. – how each is generated.– the mechanism by which each replaces hydrogen.



• All EAS reactions occur by a three-step mechanism.– Step 1: Generation of the electrophile.

– Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond.



– Step 3: Take a proton away. Proton transfer regenerates the aromatic ring.


Chlorination and Bromination

– Step 1: Formation of the electrophile (a chloronium ion).



– Step 3: Take a proton away. Proton transfer to FeCl4– forms HCl, regenerates the Lewis acid catalyst, and gives chlorobenzene.


Nitration

• The electrophile, NO2+, generated in two steps.

• Step 1: Add a proton.

• Step 2: Break a bond to form a stable molecule or ion.


Friedel-Crafts Alkylation

• Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group.



– Step 1: Formation of an electrophile.




– Step 3: Take a proton away. Proton transfer regenerates the aromatic ring.


Friedel-Crafts Alkylations

• There are two major limitations on F-C alkylations.– It is practical only with stable carbocations, such as 2°

and 3° carbocations.– It fails on benzene rings bearing one or more of these

strongly electron-withdrawing groups.


Friedel-Crafts Acylations

• Treating an aromatic ring with an acid chloride in the presence of AlCl3.– Acid (acyl) chloride: a derivative of a

carboxylic acid in which the -OH is replaced by a chlorine.


Friedel-Crafts Acylations

– Step 1: Formation of the electrophile.


Other Benzene Alkylations

• Generation of carbocations– Treating an alkene with a protic acid, most commonly

H2SO4 or H3PO4.


Other Benzene Alkylations

• Generation of carbocations– Treating an alcohol with H2SO4 or H3PO4.


Di- and Polysubstitution

• Existing groups on a benzene ring influence further substitution in both orientation and rate.

• Orientation– Certain substituents direct new substitution

preferentially toward ortho-para positions, others direct preferentially toward meta positions.

• Rate– Certain substituents are activating toward further

substitution, others are deactivating toward further substitution.



– -OCH3 is ortho-para directing.



– -NO2 is meta directing.



• Generalizations– 1. Alkyl groups, phenyl groups, and substituents

in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing; all other substituents are meta directing.

– 2. All ortho-para directing groups are activating toward further substitution; the exceptions to this generalization are the halogens, which are weakly deactivating.

– 3. All meta directing groups carry either a partial or full positive charge on the atom bonded to the ring.

9-47

Draw the structure of the compounds and number them in increasing order of reactivity towards SarE


Benzene toluene

aminobenzene chlorobenzene

nitrobenzene

9-48

Give the structure of the major product:

SO3H + Br2FeBr3

Br + CH3CH2CCl

O

AlCl3

NH2 +HNO3H2SO4

1

2

3


CH3O2N + KMnO4H2O

OH

O2N

+ SO3H2SO4

CCH3

O

+ CH3CHCl

CH3AlCl3

4

5

6



• The order of steps is important.


Theory of Directing Effects

• The rate of electrophilic aromatic substitution– The rate of EAS is determined by the slowest

step in the reaction.– For almost every EAS, the rate-determining

step is attack of E+ on the aromatic ring to give a resonance-stabilized cation intermediate.

– The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction.



• For ortho-para directors, ortho-para attack forms a more stable cation than meta attack.– Ortho-para products are formed faster than

meta products.

• For meta directors, meta attack forms a more stable cation than ortho-para attack.– Meta products are formed faster than ortho-

para products.



– -OCH3; assume meta attack.



– -OCH3; assume para (or ortho) attack.



– -NO2; assume meta attack.



– -NO2; assume para attack.

– Contributor (e) places positive charge on adjacent atoms.

9-57

Synthesize the compound below starting from benzene and provide all the necessary reagents needed (a-d) and the intermediate product (e).


SO3H

CH3

+

(a)

(b)

(e)

(c)

(d)


Activating-Deactivating

• Any resonance effect such as those of -NH2, –OH, and –OR, which delocalizes the positive charge on the cation intermediate, lowers the activation energy for its formation and activates the ring toward further EAS.

• Any resonance or inductive effect such as those of –NO2, –C=O, -SO3H, –NR3

+, –CCl3, and –CF3, which decreases electron density on the ring, deactivates the ring toward further EAS.


Halogens

• Halogens: the resonance and inductive effects operate in opposite directions.– The inductive effect: halogens have an electron-

withdrawing inductive effect; therefore, aryl halides react more slowly in EAS than benzene.

– The resonance effect: a halogen ortho or para to the site of electrophilic attack stabilizes the cation intermediate by delocalizing the positive charge; halogen, therefore, is ortho-para directing.


Phenols

• The functional group of a phenol is an -OH group bonded to a benzene ring.


Phenols

– Some phenols


Acidity of Phenols

• Phenols are significantly more acidic than alcohols.


Acidity of Phenols

The greater acidity of phenols compared with alcohols is the result of the greater stability of the phenoxide ion relative to an alkoxide ion.


Acidity of Phenols

Ring substituents, particularly halogens and nitro groups, increase the acidity of phenols by a combination of resonance and inductive effects.


Acidity of Phenols

• Phenols are weak acids.– They react with strong bases to form water-

soluble salts.

– They do not react with weak bases, such as sodium bicarbonate.


Acidity of Phenols

• Separations


Phenols as Antioxidants

• Autoxidation– An oxidation requiring O2 and no other

reactant.– A radical chain process.– Converts an R-H to R-O-O-H, a

hydroperoxide.

• We are concerned with allylic autoxidation.– Allylic carbon: A carbon adjacent to a C=C.



– Step 1: Chain initiation: Formation of radicals from nonradicals.



• Steps 2a & and 2b Chain propagation: Reaction of a radical to form a new radical.– Step 2a

– Step 2b:



– Vitamin E is a

natural antioxidant.

– BHT and BHA are

synthetic antioxidants.


Benzene and its Derivatives

End Chapter 9

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